1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls that include a center, an envelope layer, a mid layer, and a cover.
2. Description of the Related Art
Golf players'foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.
In light of achieving various performance characteristics, golf balls each having a multilayer structure have been proposed. JPH9-56848/JP No. 2888197 (U.S. Pat. No. 5,725,442), JPH8-336618/JP No. 2817668 (U.S. Pat. No. 5,733,205), and JPH10-328326/JP No. 3985107 (U.S. Pat. No. 6,468,169) each disclose a golf ball that includes a core, an envelope layer, an inner cover, and an outer cover. JP2004-130072/JP No. 4214003 (US2004/0029648) discloses a golf ball that includes a core and a cover. The core has a three-layer structure. The principal component of the cover is a polyurethane thermoplastic elastomer. JP2001-17575/JP No. 3525813 (U.S. Pat. No. 6,271,296) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover.
In a golf ball that includes a core consisting of a center and an envelope layer, a large and hard center may be used. Spin is easily provided to this golf ball. In addition, the energy loss is high, and thus the resilience performance deteriorates. Therefore, even when this golf ball is hit with a long iron or a middle iron, a sufficient flight distance is not obtained.
An object of the present invention is to provide a golf ball that provides a large flight distance when being hit with a long iron or a middle iron.